Magnetostrictive torque sensor shafts of the type having a magnetostrictive metal layer of a chevron configuration formed on the surface of a round bar-shaped shaft base material by plating, thermal spraying or otherwise which functions as a magnetically anisotropic portion have been known as disclosed in, for example, Japanese Patent Application Laid-Open Nos. 59-164931 and 3-68826.
Such a torque sensor shaft of the type having a magnetostrictive metal layer formed over the entire peripheral surface of a rod-shaped shaft base material has also been known as described in, for example, Japanese Patent Application Laid-Open No. 4-359127.
A different type of magnetostrictive torque sensor shaft having a magnetically anisotropic portion in the form of grooves formed by knurling on the surface of the shaft base material has been known as described in, for example, Japanese Patent No. 169326.
Further, there have been known magnetostrictive torque sensor shafts of the type having such knurl grooves subjected to shot peening thereby to attain hysteresis reduction and sensitivity improvement when the shaft is incorporated into a torque sensor, so as to insure stable sensor performance, as taught in, for example, Japanese Patent Application Laid-Open No. 2-221830.
Unfortunately, torque sensor shafts having a magnetostrictive metal layer of chevron configuration formed by plating and/or thermal spraying on the surface of the shaft base material involve a disadvantage that masking and etching operations are required for the formation of the magnetostrictive layer, which requires a complex process. Another problem is that torque sensors using such a shaft have no sufficient sensitivity justifiable from the standpoint of practical application. A further problem is that the magnetostrictive metal layer formed by plating and/or thermal spraying has a drawback in respect of service durability.
With torque sensor shafts having a magnetostrictive metal layer formed over the entire periphery of the shaft base material, it is pointed out that the step of imparting magnetic anisotropy to the magnetostrictive metal layer involves some difficulty and, in addition, the magnetostrictive metal layer lacks service durability.
Torque sensor shafts having knurl grooves formed on the surface of the base material have no sufficient sensitivity even if any strictly selected shaft material is used for the shaft.
In contrast to those having grooves formed merely by knurling, shafts having knurl grooves subjected to shot peening can be advantageously used to provide a torque sensor having good sensitivity characteristics which is suitable for practical application. However, when a torque sensor having further improved sensitivity is required in response to the demands of growing markets for sensors, such a shaft is still insufficient to meet the requirement.
In Japanese Patent Application Laid-Open No. 3-282338 there is disclosed a method which comprises forming knurl grooves on the surface of the shaft base material, then forming a magnetostrictive metal layer on the surface of the knurl grooves, thereby to develop magnetic anisotropy on the surface of the shaft. According to such arrangement, high sensitivity measurement of applied torque can be reasonably expected.
However, with a magnetostrictive metal layer simply formed on the knurl grooves, it is impracticable to expect that the strength of the magnetostrictive metal layer is sufficient to withstand applied torque. In particular, in order to enable accurate measurement of a minute torque, it is necessary that the shaft material be configured to be of relatively small diameter so that the magnetically anisotropic portion can exhibit sufficient change in magnetic permeability in response to the torque applied. However, when an overload acts on a shaft of such a small diameter, the strength of the magnetostrictive metal layer falls short of the requirement, possibly resulting in degradation in sensor characteristics of the torque sensor comprised of the shaft.